Compressed gas regulators operate by reducing a high pressure gas through a variable orifice to a lower delivery pressure. As the pressure of the source drops, the regulator's control valve is automatically adjusted by a pressure sensing element to maintain a constant discharge pressure. The pressure sensing element can be a piston or a flexible diaphragm. Flexible diaphragms are generally preferred for low discharge pressures (less than 200 psig) because they provide more sensitive pressure control compared to pistons, which are rigid diaphragms, of equal area. In order to function adequately, a diaphragm must be thin enough to flex at the operating pressure of the regulator. This also means it must be highly stressed under normal operating conditions.
A major operational problem of flexible diaphragm regulators is rupture of the diaphragm, which can cause loss of the gas into the atmosphere. Diaphragms rupture for various reasons including: weakening caused by fatigue, buckling caused by overpressurization, and loss of strength due to corrosion. Whatever the cause, a ruptured diaphragm results in the loss of pressure control as well as a discharge through the regulator's bonnet. This discharge is not a safety problem for ordinary inert gases, such as nitrogen, argon, or helium, provided that local ventilation is adequate to prevent asphyxiation. However, discharge of toxic, flammable, or corrosive gases into the working area is extremely dangerous. Gases can also escape through the bonnet by leaking through the diaphragm seal. Although this type of discharge is slower than one caused by a diaphragm rupture, it still can be a safety problem for flammable or toxic gases.
Several approaches have been used to manage gas discharges caused by ruptured or leaking diaphragms. These include: captured vent bonnets, piston pressure sensing elements, double diaphragm, and piston-backed diaphragms. Captured vent bonnets are the most common method of managing gas releases caused by diaphragm failures. In a captured vent regulator, the feed-through for the adjusting knob on the regulator is sealed so as to be gas tight, and a vent line is provided from the bonnet to carry away any leakage. This type of regulator has the benefit of simplicity, but requires installation of a separate vent line that must be piped to a safe discharge location.
A piston controlled regulator is essentially the same as a flexible diaphragm regulator except that the flexible diaphragm is replaced by a piston capable of withstanding full cylinder pressure. The piston moves in the regulator, rather than flexing, and has a dynamic seal, such as an O-ring, between it and the inside wall of the bonnet. While the piston, which is rigid, does not rupture, leaks can occur at the O-ring seal. Overall, the reliability of the piston is higher than the flexible diaphragm with respect to leaks into the bonnet. The major disadvantage of the piston controlled regulator is low sensitivity relative to a flexible diaphragm of equal area. To compensate for this, a much larger piston diameter is required relative to a flexible diaphragm controlled regulator. This makes the piston controlled regulator large in size and more expensive than the flexible diaphragm controlled regulator.
A double flexible diaphragm regulator, such as shown in U.S. Pat. Nos. 1,042,745 to E. Zahm, issued Oct. 29, 1912; 2,035,151 to G. P. Eches, issued Mar. 24, 1936; 2,147,850, to A. D. MacLean, issued Feb. 21, 1939; and 4.010,769, to B. L. DeLorenzo et al, issued Mar. 8, 1977 include a pair of spaced flexible diaphragms with a chamber therebetween. However, these regulators either do not include means for determining when a diaphragm has failed, or vents the leaking gas to the atmosphere. The later can be dangerous if the gas is toxic or flammable. As shown in U.S. Pat. Nos. 2,691,773, to H. V. Lichtenberger, issued Oct. 12, 1954, 3,131,638, to L. F. Wilson et al, issued May 5, 1964 and 3,661,060, to J. C. Bowen, issued May 9, 1972, there are double diaphragm devices which have means for detecting a leak through one of the diaphragms. However, these devices are not pressure regulators. Also, although a diaphragm controlled pressure control device having means for detecting failure of the diaphragm is shown in the U.S. Pat. No. 4,272,959, to K. Yamane, issued June 16, 1981, this device is complex and expensive to manufacture.
The piston-backed, flexible diaphragm controlled regulator overcomes the sensitivity problem of the piston controlled regulator and the reliability problem of a flexible diaphragm controlled regulator. In this type of regulator, a flexible diaphragm and piston are combined in series with the piston contacting the diaphragm on the low pressure side thereof. The flexible diaphragm provides sensitive pressure control, while the backup piston prevents leaks through the bonnet should the flexible diaphragm fail. Both the diaphragm and piston seals must simultaneously fail in order for gas to escape through the bonnet. In addition to high reliability of the pressure sensing element, the piston and diaphragm regulator preserves operability during a diaphragm failure. One disadvantage of the piston-backed diaphragm regulator is that there is no external indication of a diaphragm failure. Except for subtle changes in operating characteristics, the user would not be aware that he is operating with the backup piston. Thus, failure of the O-ring seal on the piston would allow gas to vent through the bonnet. Therefore, it would be desirable to have a piston-backed flexible diaphragm pressure regulator which would indicate failure of the flexible diaphragm to allow the user to take necessary steps to fix the regulator before the piston seal began to leak.